Centering sensor and indicator system

ABSTRACT

A system for sensing and indicating the centering of a sheet continuously fed through a machine includes a pair of laterally spaced apart sensing arms biased into contact with the opposite edges of the sheet as the sheet is being fed through the machine. Separate transducers responsive to movement of the sensing arms produce electrical signals representing the actual position of each edge of the sheet. The signals from both transducers are compared with input signals representing the known centered position of the sheet to provide an output indicating the magnitude and direction of the deviation from center independently of variations in the width of the sheet or any lateral movement of the sheet through the machine. An alarm is activated when the sheet deviates from its centered position by more than a selected amount.

United States Patent McCann Apr. 23, 1974 Primary Examiner-Allen N. Knowles Assistant Examiner-Gene A. Church [75] Inventor: Gary D. Mccann, Glendale Calm Attorney, Agent, or Firm-Christie, Parker & Hale [73] Assignees: Ed ilvin J aeger, Pasadena, Calif.; 57 ABSTRACT gyP fi' s i g g f 3 A system for sensing and indicating the centering of a p l e S sheet continuously fed through a machine includes a [22] Filed: Mar. 21, 1973 pair of laterally spaced apart sensing arms biased into contact with the opposite edges of the sheet as the Appl' 3430l1 sheet is being fed through the machine. Separate transducers responsive to movement of the sensing [5 2] US. Cl. 226/19 arms produce electrical signals representing the actual [51] Int. Cl. B65h 25/26 position of each edge of the sheet. The signals from [5 8] Field of Search 226/15, 17, 18-23; both transducers are compared with input signals rep- 198/194, 202 resenting the known centered position of the sheet to provide an output indicating the magnitude and direc- [56] References Cited tion of the deviation from center independently of var- UNITED STATES PATENTS iations in the width of the sheet or any lateral move- 3 368 726 M1968 Funk at a] 26/17 ment of the sheet through the machine. An alarm is 3 323 699 6/1967 Bricker....::1::::::::::::::::::226/15 activated when the Sheet deviates from its mered position by more than a selected amount. 22 Claims, 4 Drawing Figures 68 Mfffe #AZJFM BACKGROUND OF THE INVENTION This invention relates to a system for sensing the centering of work continuously fed through a machine, and for indicating the deviation of the work from a known centered position independently of the width of the work or its lateral movement through the machine. 7

There are industrial applications in which work is continuously fed through a machine in a centered position. For example, in the carpet industry a mat of woven jute is continuously unrolled and fed through a tufting machine. The jute provides the primary backing for a carpet, and the tufting machine inclu-des reciprocating needles which automatically thread strands of yarn into the jute to produce appropriately designed tufting for the carpet.

Typically, the tufting covers a major portion of the jute backing sheet. A selvedge (section which has no tufting) several inches in width is left along each marginal edge of the jute. The selvedge strips are then engaged with pins carried on a chain conveyor to feed the tufted jute backing sheet through an oven in which latex and a second backing sheet are applied to the jute. The two selvedge strips are then trimmed to provide the finished carpet. The selvedge strips usually are trimmed by feeding the carpet past a pair of fixed knife blades spaced apart by a distance equal to the width of the finished carpet.

It is of critical importance to maintain the jute backing sheet in a continuous on-center position when tufting is applied to it. If the tufting deviates from a centered position on the jute, a blank untufted area will be present at the edge of the finished carpet after the selvedge strips are trimmed. A carpet roll having such untufted areas is considered in the industry to be a factory second, and generally can be sold only at such a substantial discount that money will be lost by the carpet manufacturer.

The manufacturer can insure against blank areas being present at the edges of the finished carpet by adding several inches of excess tufting. However, even a few extra needles added to the tufting operation can cost the manufacturer an extra several hundred dollars per day per machine. If the carpet can be kept reasonably on-center during tufting operations, less excess yarn is needed, which can provide the manufacturer a substantial cost savings.

Generally, the operator of the tufting machine visually monitors the centering of the jute backing sheet. This task is difficult, though, because the two edges of the jute are spaced a relatively long distance apart (about 150 inches). In one prior art method of mechanically monitoring the centering of the jute, a microswitch located along one edge of the jute activates an alarm whenever the width of the selvedge strip exceeds a predetermined amount. However, this method has not proved to be practical, because the width of the jute backing sheet usually varies several inches between one place and another in a given roll, and this variation in width will cause errors in the centering of the tufting. Moreover, many carpets are made by constantly shift-ing the jute back and forth laterally to apply the tufting in a zigzag pattern. This lateral shifting of the jute complicates the process of determining, either mechanically or visually, when the tufting is off-center.

SUMMARY OF THE INVENTION Briefly, this invention provides a system for sensing and indicating the centering of work continuously fed through a machine in which the work has a known centered position. The system includes means for sensing the instantaneous position of each edge of the work as it moves through the machine. The sensing means produces a first output representing the position of one edge, and a second output representing the position of the other edge. The first and second outputs are combined to produce a third output representing the instantaneous deviation of the work from its centered position. The third output is then fed to a meter or used to activate an alarm or the like to indicate the deviation of the work from its centered position.

In a preferred form of the invention, the sensing means comprise a pair of sensing arms continuously engaged with the opposite longitudinal edges of the work independently of variations in the width of the work or lateral shifting of the work as it travels through the machine. Thus, the sensing and indicating system is especially suitable for use in the carpet industry, because the movement of the sensing arms can provide indications of the instantaneous position of each edge of a jute backing sheet independently of the usual variations in its width or lateral reciprocating motion of the jute when tufting is applied to it in a zigzag pattern. Transducers controlled by movement of the sensing arms produce output signals which are compared with input v signals representing the known centered position of the jute to provide an output indicating the magnitude and direction of the deviation from center. The output then can be fed to an alarm to be activated when the jute deviates from its true centered position by a selected amount.

These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary perspective view showing the centering sensing and indicating system of this invention used in combination with a carpet tufting machine;

FIG. 2 is an enlarged fragmentary elevation view showing an edge position sensing device used in the system of FIG. 1;

FIG. 3 is an electrical schematic diagram of the cen tering and indicating system of this invention; and

FIG. 4 is a schematic elevation view showing a meter for indicating the instantaneous deviation of the carpet shown in FIG. 1 from a known centered position in the carpet tufting machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT is continuously fed in a known centered position. A

sheet 12 of jute is continuously unrolled and fed in a longitudinal direction through the tufting machine. The jut is fed under a vertically reciprocating needle bar 14.

Separate strands 16 of yarn are threaded through respective vertically disposed needles 18 carried by needle bar 14. Push rods 20 connected to needle bar 14 are driven up and down by a drive mechanism (not shown) to reciprocate the needle bar and stitch yarn 16 into the traveling jute backing sheet 12 to provide tufting for the jute.

The tufting is commonly stitched in a zigzag pattern as shown at 22. The zigzag pattern is formed by continuously shifting jute l2 laterally back and forth as it travels longitudinally under needle bar 14. The jute is shifted laterally by a reciprocating drive mechanism which includes an elongated step-over bar 24 extending over the jute. The step-over bar is disposed in the plane parallel to and closely spaced apart downstream of needle bar 14. A drive arm 26 connected to one end of step-over bar 24 is driven back and forth laterally by suitable drive means (not shown) to reciprocate stepover bar 24.

An elongated pin roll 28 carried below step-over bar 24 is held tightly in contact with the top surface of the traveling jute backing sheet 12. The pin roll reciprocates laterally with the step-over bar to shift the jute back and forth during its travel under needle bar 14 so the tufting is applied in an appropriate zigzag pattern.

Selvedge strips 30 are provided along the marginal edges of the jute.

The tufting is maintained in its centered position on the jute by a centering and indicating system which includes a pair of edge sensors 32 for continuously sensing the position of opposite longitudinal edges 31 of jute backing sheet 12. As shown best in FIG. 2, each sensor 32 includes a vertically extending, pendulously supported elongated sensing arm 34 biased into contact with a respective edge 31 of the jute. Preferably, each sensing arm is held in contact with the jute as close to the vertical plane of the needles 18 as possible, because the on-center position of the jute at the point where the tufting is applied is the parameter of importance. Each sensing arm 34 is carried on the end of an elongated shaft 36 extending outwardly from the front end of a rotatable cylindrical housing 38 mounted above a respective edge 31 of the jute. Each shaft 36 rotates about a horizontal axis aligned in the same vertical plane as the corresponding edge 31 spaced below it. A corresponding drum 40 inside each housing 38 is engaged with shaft 36, and a spring 42 is wound on the drum to apply a constant bias to the bottom of the sensing arm to hold it in continuous contact with edge 31 of the jute. Rotation of shaft 36 in response to movement of sensing arm 34 moves a wiper arm 43 (represented schematically in FIG. 3) of a potentiometer (represented schematically at 44 in FIGS. 2 and 3) located in and fixed to housing 38. The resistance element of potentiometer 44 and wiper arm 43 are connected to an electrical connector (represented schematically at 45) in housing 38.

Each sensor 32 is supported by a separate rotating rod 46 extending into the rear of housing 38. Each rod 46 is journaled in a corresponding bearing 48 and is driven by step-over bar 24 (in a manner described in detail below). During operation of the tufting machine, lateral movement of the jute moves the sensing arm which rotates shaft 36. The shaft drives wiper arm 43, and rod 46 drives the resistance element of the potentiometer independently of the wiper arm. Rod 46 is fixed to housing 38 and causes the housing and the potentiometer resistance element 44 to rotate as a unit in response to the lateral movement of the step-over bar. The housing does not rotate when the jute step-over bar is not used, but the sensing arms continue to follow movement of the jute.

The end of each rod 46 is connected to a separate upright lever arm 50, and both lever arms are interconnected by a pair of horizontally disposed, collinear cross-bars 52 extending laterally above jute 12. Crossbars 52 are mutually connected to an upright link arm 54 located above the center of the jute. The bottom of link arm 54 is connected to a reciprocating drive linkage operated by the reciprocating motion of step-over bar 24. The linkage includes a horizontally disposed elongated drive arm 56 fixed at one'end to the stepover bar and having its opposite end connected to the bottom of an upright lever arm 58. An elongated rotation arm 60 journaled in a bearing 62 connects the top of lever arm 58 with the bottom of link arm 54.

In use, the reciprocating travel of step-over bar 24 causes the linkage to reciprocate cross-bars 52 which, in turn, rotate rod 46 and cause sensor housing 38 and its corresponding potentiometer element 44 to follow the laterally shifting motion of the step-over bar. Springs 42 constantly bias sensing arms 34 against longitudinal edges 31 of the jute so the edges of the jute can independently drive the wiper arms 43 of potentiometers 44. As the jute is shifted back and forth during tufting operations, wiper arms 43 and potentiometer elements 44 move back and forth, but there is no relative movement between each wiper arm 43 and its corresponding resistance element as long as the jute remains centered. In essence, the movement of potentiometer elements 44 in response to the step-over bar movement marks the desired on-center position of the jute. The movement of wiper arms 43 in response to movement of sensing arms 34 marks the actual position of the jute. Each potentiometer produces an output voltage of a given magnitude when the jute is oncenter. If the jute strays off-center, the voltage signals vary in accordance with the size of the deviation, and its direction, right or left, from a given reference point.

Electrical leads and 72 from the wiper arms of sensors 32 are fed to a meter relay 68 which indicates the deviation of the jute from its known centered position. The operation of meter relay 68 is understoo best by referring to FIGS. 3 and 4. FIG. 3 shows an electrical schematic diagram of the meter relay which preferably comprises a Compack IV Solid State Controller manufactured by API Instruments Company, Chesterland, Ohio. The meter relay is operated by a DC input voltage produced by connecting potentiometer 44 of sensors 32 into a bridge arrangement illustrated in FIG. 3. Wiper arms 43 are coupled with corresponding lead wires 70 and 72 which provide the voltage input to meter relay 68.

In use, the meter relay is calibrated by initially centering jute backing sheet 12 on the tufting machine and then adjusting sensing arms 34 and their associated linkage so that the voltage signal carried by lead wire 70 is offset by the voltage signal in lead wire 72, thereby resulting in a zero input voltage to the meter relay. Thereafter, as the jute travels through the tufting machine, lateral movement of the jute will cause wiper arms 43 to move back and forth to indicate the actual position of the edges of the jute. The step-over bar will rotate rods 46 to move potentiometer resistance elements 44 back and forth indicating the true on-center position of the jute. As long as the backing sheet remains centered on the tufting machine, the meter relay will cause the voltage of one potentiometer to be offset by the voltage of the other, and will produce a zero voltage which is fed to a meter 74. The zero voltage will produce a neutral scale reading on meter 74, indicating that the jute is on-center.

If the jute backing sheet goes off-center, a difference in potentiometer voltages is developed by the bridge circuit. This voltage difference is fed to a voltage divider and DC amplifier circuit (represented schematically at 76) which produces an output voltage at 78 proportional to the magnitude of the voltage difference developed across the bridge. The polarity of voltage signal 78 will depend upon which edge of the jute is moving away from an imaginary central longitudinal axis through the tufting machine. Voltage signal 78 is fed to meter 74 to drive an indicator needle 80 (see FIG. 4) which provides a continuous scale reading of the size of the deviation (in inches) from the on-center position, and the direction of the deviation from the imaginary central axis. 4

By increasing or decreasing voltage V,, the gain across the bridge circuit can bechanged so the meter scale factor can be adjusted to read the desired number of inches in deviation for a given voltage input.

Meter relay 68 also includes a potentiometer 82 having a high set point wiper and related circuitry (represented schematically at 84) and a low set point wiper and related circuitry (represented schematically at 86). The position of the high set point wiper is selectively adjusted by a knob 88 on the control panel of the meter relay to produce an output voltage signal at 90 proportional to the desired maximum allowable deviation of one longitudinal edge of the jute from the on-center position. Similarly, the position of the low set point wiper is selectively adjusted by a knob 92 to produce an output voltage signal at 94 proportional to the desired maximum allowable deviation of the other longitudinal edge of the jute from the on-center position. The position of the high set point wiper is indicated by a needle 96 on meter 74, and a needle 98 on the meter indicates the position of the low set point wiper.

Voltage signal 78 is fed to both a high set point comparator 100 and a low set point comparator 102. Comparator 100 compares voltage 78 with wiper voltage 90 and produces an output voltage signal at 104 when the comparison of the two signals indicates that one edge of the jute has exceeded its allowable deviation from center. Similarly, comparator 102 compares voltage 78 with low set point wiper voltage 94 and produces an output voltage signal at 106 when the comparison of the two signals indicates that the other edge of the jute has exceeded its allowable deviation. Signal 104 energizes a relay 108 which activates a suitable alarm 110 to indicate that one edge of the jute has exceeded its allowable deviation. Signal 106 energizes a relay 112 to activate a second alarm indicating that the other edge of the jute has exceeded its allowable deviation. Preferably, alarms 110 and 114 comprise warning lights (shown in FIG. 4) mounted on the meter relay control panel, although the alarms can take other forms such as audible warning devices, or the like.

Thus, the present invention provides a system for continuously sensing the position of work continuously fed through a machine along a known centered position. Sensing arms 34 provide means for constantly sensing the location of the opposite edges of the work, and the deviation of the work from its on-center position is determined independently of any lateral movement of the work through the machine or variations in the width of the work. Moreover, the output generated by the sensing arms can be used to drive a closed loop system for controlling the position of the jute backing sheet in the tufting machine. For example, the actual deviation of the jute can be used as a control system input for automatically adjusting the lateral position of the roll on which the jute is wound to keep the jute oncenter.

I claim: 1. In a machine through which work is fed in a longitudinal direction, the work having a width defined by a pair of spaced apart longitudinal edges, and in which the machine includes lateral shifting means for moving the work laterally back and forth relative to an imaginary axis in the machine as the work is being fed in said longitudinal direction, the work having a desired oncenter position which moves laterally back and forth relative to the imaginary axis in response to lateral shifting of the work, apparatus for use in maintaining the work on its desired on-center position as the work shifts laterally and moves longitudinally through the machine, the apparatus comprising means for sensing the actual position of each longitudinal edge of the work relative to the imaginary axis, independently of variations in the width of the work, as the edges are being shifted laterally back and forth by the lateral shifting means during longitudinal movement of the work through the machine, means responsive to movement of the lateral shifting means for sensing the desired on-center position of the work relative to the imaginary axis as the work is being shifted laterally back and forth, and

means for comparing the sensed actual position of each longitudinal edge with the sensed on-center position of the work to produce an output representative of the lateral deviation of the work from its desired oncenter position.

2. Apparatus according to claim 1 in which the edge sensing means includes first and second sensing arms biased into contact with the longitudinal edges of the work to follow lateral movement of the work.

3. Apparatus according to claim 2 in which the machine includes means for continuously feeding work in the form of an elongated continuous sheet in a longitudinal direction through the machine, and in which the sensing arms are continuously biased into contact with the edges of the continuous sheet independently of the lateral movement of the sheet or of variations in the width of the continuous sheet.

4. Apparatus according to claim 3 in which the machine includes means traversing the sheet and lying in a known plane for working on the sheet, and in which the sensing arms are disposed in substantially the same plane.

5. Apparatus according to claim 2 including means responsive to lateral movement of the first sensing arm for cooperating with the on-center sensing means to produce a first output representative of the lateral distance between one longitudinal edge and the desired on-center position, means responsive to lateral movement of the second sensing arm for cooperating with the on-center sensing means to produce a second output representative of the lateral distance between the other longitudinal edge and the desired on-center position, and means for comparing the first and second outputs to produce an output representative of the acutal lateral deviation of the work from its desired on-center position.

6. Apparatus according to claim including first and second electrical elements movable in response to lateral movement of the first and second sensing arms, and third and fourth electrical elements movable in response to lateral movement of the lateral shifting means to indicate the desired on-center position of the work, the relative movement of the first and third electrical elements cooperating to produce a first electrical output, the relative movement of the second and fourth electrical elements cooperating to produce a second electrical output, the first and second electrical outputs being compared to produce an electrical output signal representative of the lateral deviation of the work from its desired on-center position.

7. Apparatus according to claim 6 in which the lateral shifting means includes an elongated laterally extending step-over bar having means thereon engaged with the work, drive means engaged with the step-over bar for shifting the bar laterally to move the work later? ally back and forth as it passes longitudinally through the machine, the third electrical element being connected with one end of the step-over bar and the fourth electrical element being connected with the other end of the step-over bar so the third and fourth electrical elements follow the lateral shifting movement of the bar, the first and second sensing arms being adapted to drive the first and second electrical elements, respectively, so they follow the laterally shifting positions of the longitudinal edges of the work.

8. Apparatus according to claim 5 including means for selecting an allowable lateral deviation of the work from its on-center position, means for comparing the actual lateral deviation of the work from its on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work relative to its selected deviation.

9. Apparatus according to claim 8 including means for selecting a first allowable lateral deviation of one longitudinal edge from the on-center position, means for selecting a second allowable lateral deviation of the other longitudinal edge from the on-center position, first comparison means for comparing the first selected deviation with the actual deviation of its corresponding edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of its corresponding edge from the on-center position, and means for indicating the outputs of the first and second comparison means.

10. Apparatus according to claim 9 including first alarm means to be activated when the actual lateral deviation of said one edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual lateral deviation of said other edge exceeds the second selected allowable deviation.

11. Apparatus according to claim 1 including means for selecting an allowable lateral deviation of the work from its desired on-center position, means for comparing the actual lateral deviation of the work from its desired on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work from its on-center position relative to the selected deviation.

12. Apparatus according to claim 11 including means for selecting a first allowable lateral deviation of one longitudinal edge from the desired on-center position, means for selecting a second allowable lateral deviation of the other longitudinal edge from the desired oncenter position, first comparison means for comparing the first selected deviation with the actual deviation of its corresponding edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of its corresponding edge from the on-center position, and means for indicating the outputs of the first and second com parison means.

13. Apparatus according to claim 12 including first alarm means to be activated when the actual lateral deviation of said one edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual deviation of said other edge exceeds the second selected allowable deviation.

14. Apparatus according to claim 1 including means responsive to said output for indicating the deviation of the work from its desired on-center position, and in which the indicating means indicates the lateral direction of said deviation.

15. Apparatus for sensing the on-center position of a continuous work sheet fed in a longitudinal direction through a machine for working on the continuous sheet and in which the position of the sheet is continuously being shifted laterally back and forth as the sheet is being fed longitudinally through the machine, the continuous sheet having a desired on-center position which moves laterally back and forth in response to the lateral shifting of the sheet, the continuous sheet having a width defined by laterally spaced apart first and second longitudinal edges, the apparatus comprising first and second movable sensing arms held in contact with the first and second longitudinal edges, respectively, of the continuous sheet to follow the actual lateral back and forth movement of the first and second edges, independently of variations in the width of the sheet, as the sheet is being shifted laterally back and forth during its passage through the machine,

means responsive to movement of the first sensing arm for producing a first input representative of the actual position of the first edge of the sheet, means responsive to movement of the second sensing arm for producing a second input representative of the acutal position of the second edge of the sheet,

means for sensing the desired on-center position of the sheet as the sheet is being shifted laterally back and forth during passage through the machine, and

means for comparing the first and second inputs with the sensed on-center position of the sheet to produce an output representative of the lateral deviation of the sheet from its desired on-center position.

16. Apparatus according to claim including means responsive to said output for indicating the lateral deviation of the sheet from its on-center position, and in which the indicating means indicates the direction of said lateral deviation.

17. Apparatus according to claim 15 in which the first input producing means cooperates with the on center sensing means to produce a first output representative of the lateral distance between the first longitudinal edge and said desired on-center position, and in which the second input producing means cooperates with the on-center sensing means to produce a second output representative of the lateral distance between the second longitudinal edge and the desired on-center position, and including means for comparing the first and second outputs to produce the output representing the lateral deviation of the work from its desired oncenter position.

18. Apparatus according to claim 17 including means for selecting an allowable lateral deviation of the work from its on-center position, means for comparing the actual deviation of the work from its on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work relative to its selected deviation.

19. Apparatus according to claim 18 including means for selecting a first allowable lateral deviation of the first longitudinal edge from the desired on-center position, means for selecting a second allowable lateral deviation of, the second longitudinal edge from the desired on-center position, first comparison means for comparing the first selected deviation with the actual deviation of the first edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of the second edge from the on-center position, and means for indicating the outputs of the first and second comparison means.

20. Apparatus according to claim 19 including first alarm means to be activated when the lateral actual deviation of the first edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual lateral deviation of the second edge exceeds the second selected allowable deviation.

21. Apparatus according to claim 15 including first and second electrical elements movable in response to the first and second sensing arms, and third and fourth electrical elements movable in response to lateral shifting of the continuous sheet to indicate the desired oncenter position of the sheet, the relative movement of the first and third electrical elements cooperating to produce a first electrical output, the relative movement of the second and fourth electrical elements cooperating to produce a second electrical output, the first and second electrical outputs being compared to produce an electrical output signal representative of the lateral deviation of the sheet from its desired on-center position. 1

22. Apparatus according to claim 21 in which the lateral shifting means includes a laterally extending elongated step-over bar having means thereon engaged with the continuous sheet, drive means engaged with the step-over bar for shifting the bar laterally to move the work laterally back and forth as its passes longitudinally through the machine, the third electrical element being connected with one end of the step-over bar and the fourth electrical element being connected with the other end of the step-over bar so the third and fourth electrical elements follow the lateral shifting movement of the bar, the first and second sensing arms being adapted to drive the first and second electrical elements, respectively, so they follow the laterally shifting positions of the longitudinal edges of the work. 

1. In a machine through which work is fed in a longitudinal direction, the work having a width defined by a pair of spaced apart longitudinal edges, and in which the machine includes lateral shifting means for moving the work laterally back and forth relative to an imaginary axis in the machine as the work is being fed in said longitudinal direction, the work having a desired on-center position which moves laterally back and forth relative to the imaginary axis in response to lateral shifting of the work, apparatus for use in maintaining the work on its desired on-center position as the work shifts laterally and moves longitudinally through the machine, the apparatus comprising means for sensing the actual position of each longitudinal edge of the work relative to the imaginary axis, independently of variations in the width of the work, as the edges are being shifted laterally back and forth by the lateral shifting means during lonGitudinal movement of the work through the machine, means responsive to movement of the lateral shifting means for sensing the desired on-center position of the work relative to the imaginary axis as the work is being shifted laterally back and forth, and means for comparing the sensed actual position of each longitudinal edge with the sensed on-center position of the work to produce an output representative of the lateral deviation of the work from its desired on-center position.
 2. Apparatus according to claim 1 in which the edge sensing means includes first and second sensing arms biased into contact with the longitudinal edges of the work to follow lateral movement of the work.
 3. Apparatus according to claim 2 in which the machine includes means for continuously feeding work in the form of an elongated continuous sheet in a longitudinal direction through the machine, and in which the sensing arms are continuously biased into contact with the edges of the continuous sheet independently of the lateral movement of the sheet or of variations in the width of the continuous sheet.
 4. Apparatus according to claim 3 in which the machine includes means traversing the sheet and lying in a known plane for working on the sheet, and in which the sensing arms are disposed in substantially the same plane.
 5. Apparatus according to claim 2 including means responsive to lateral movement of the first sensing arm for cooperating with the on-center sensing means to produce a first output representative of the lateral distance between one longitudinal edge and the desired on-center position, means responsive to lateral movement of the second sensing arm for cooperating with the on-center sensing means to produce a second output representative of the lateral distance between the other longitudinal edge and the desired on-center position, and means for comparing the first and second outputs to produce an output representative of the acutal lateral deviation of the work from its desired on-center position.
 6. Apparatus according to claim 5 including first and second electrical elements movable in response to lateral movement of the first and second sensing arms, and third and fourth electrical elements movable in response to lateral movement of the lateral shifting means to indicate the desired on-center position of the work, the relative movement of the first and third electrical elements cooperating to produce a first electrical output, the relative movement of the second and fourth electrical elements cooperating to produce a second electrical output, the first and second electrical outputs being compared to produce an electrical output signal representative of the lateral deviation of the work from its desired on-center position.
 7. Apparatus according to claim 6 in which the lateral shifting means includes an elongated laterally extending step-over bar having means thereon engaged with the work, drive means engaged with the step-over bar for shifting the bar laterally to move the work laterally back and forth as it passes longitudinally through the machine, the third electrical element being connected with one end of the step-over bar and the fourth electrical element being connected with the other end of the step-over bar so the third and fourth electrical elements follow the lateral shifting movement of the bar, the first and second sensing arms being adapted to drive the first and second electrical elements, respectively, so they follow the laterally shifting positions of the longitudinal edges of the work.
 8. Apparatus according to claim 5 including means for selecting an allowable lateral deviation of the work from its on-center position, means for comparing the actual lateral deviation of the work from its on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work relative to its selected deviation.
 9. Apparatus according to claim 8 including means for selecting a first aLlowable lateral deviation of one longitudinal edge from the on-center position, means for selecting a second allowable lateral deviation of the other longitudinal edge from the on-center position, first comparison means for comparing the first selected deviation with the actual deviation of its corresponding edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of its corresponding edge from the on-center position, and means for indicating the outputs of the first and second comparison means.
 10. Apparatus according to claim 9 including first alarm means to be activated when the actual lateral deviation of said one edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual lateral deviation of said other edge exceeds the second selected allowable deviation.
 11. Apparatus according to claim 1 including means for selecting an allowable lateral deviation of the work from its desired on-center position, means for comparing the actual lateral deviation of the work from its desired on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work from its on-center position relative to the selected deviation.
 12. Apparatus according to claim 11 including means for selecting a first allowable lateral deviation of one longitudinal edge from the desired on-center position, means for selecting a second allowable lateral deviation of the other longitudinal edge from the desired on-center position, first comparison means for comparing the first selected deviation with the actual deviation of its corresponding edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of its corresponding edge from the on-center position, and means for indicating the outputs of the first and second comparison means.
 13. Apparatus according to claim 12 including first alarm means to be activated when the actual lateral deviation of said one edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual deviation of said other edge exceeds the second selected allowable deviation.
 14. Apparatus according to claim 1 including means responsive to said output for indicating the deviation of the work from its desired on-center position, and in which the indicating means indicates the lateral direction of said deviation.
 15. Apparatus for sensing the on-center position of a continuous work sheet fed in a longitudinal direction through a machine for working on the continuous sheet and in which the position of the sheet is continuously being shifted laterally back and forth as the sheet is being fed longitudinally through the machine, the continuous sheet having a desired on-center position which moves laterally back and forth in response to the lateral shifting of the sheet, the continuous sheet having a width defined by laterally spaced apart first and second longitudinal edges, the apparatus comprising first and second movable sensing arms held in contact with the first and second longitudinal edges, respectively, of the continuous sheet to follow the actual lateral back and forth movement of the first and second edges, independently of variations in the width of the sheet, as the sheet is being shifted laterally back and forth during its passage through the machine, means responsive to movement of the first sensing arm for producing a first input representative of the actual position of the first edge of the sheet, means responsive to movement of the second sensing arm for producing a second input representative of the acutal position of the second edge of the sheet, means for sensing the desired on-center position of the sheet as the sheet is being shifted laterally back and forth during passage through the machine, and means for comparing the first and second inputs wiTh the sensed on-center position of the sheet to produce an output representative of the lateral deviation of the sheet from its desired on-center position.
 16. Apparatus according to claim 15 including means responsive to said output for indicating the lateral deviation of the sheet from its on-center position, and in which the indicating means indicates the direction of said lateral deviation.
 17. Apparatus according to claim 15 in which the first input producing means cooperates with the on-center sensing means to produce a first output representative of the lateral distance between the first longitudinal edge and said desired on-center position, and in which the second input producing means cooperates with the on-center sensing means to produce a second output representative of the lateral distance between the second longitudinal edge and the desired on-center position, and including means for comparing the first and second outputs to produce the output representing the lateral deviation of the work from its desired on-center position.
 18. Apparatus according to claim 17 including means for selecting an allowable lateral deviation of the work from its on-center position, means for comparing the actual deviation of the work from its on-center position with the selected allowable deviation, and means for indicating the actual lateral deviation of the work relative to its selected deviation.
 19. Apparatus according to claim 18 including means for selecting a first allowable lateral deviation of the first longitudinal edge from the desired on-center position, means for selecting a second allowable lateral deviation of the second longitudinal edge from the desired on-center position, first comparison means for comparing the first selected deviation with the actual deviation of the first edge from the on-center position, second comparison means for comparing the second selected deviation with the actual deviation of the second edge from the on-center position, and means for indicating the outputs of the first and second comparison means.
 20. Apparatus according to claim 19 including first alarm means to be activated when the lateral actual deviation of the first edge exceeds the first selected allowable deviation, and second alarm means to be activated when the actual lateral deviation of the second edge exceeds the second selected allowable deviation.
 21. Apparatus according to claim 15 including first and second electrical elements movable in response to the first and second sensing arms, and third and fourth electrical elements movable in response to lateral shifting of the continuous sheet to indicate the desired on-center position of the sheet, the relative movement of the first and third electrical elements cooperating to produce a first electrical output, the relative movement of the second and fourth electrical elements cooperating to produce a second electrical output, the first and second electrical outputs being compared to produce an electrical output signal representative of the lateral deviation of the sheet from its desired on-center position.
 22. Apparatus according to claim 21 in which the lateral shifting means includes a laterally extending elongated step-over bar having means thereon engaged with the continuous sheet, drive means engaged with the step-over bar for shifting the bar laterally to move the work laterally back and forth as its passes longitudinally through the machine, the third electrical element being connected with one end of the step-over bar and the fourth electrical element being connected with the other end of the step-over bar so the third and fourth electrical elements follow the lateral shifting movement of the bar, the first and second sensing arms being adapted to drive the first and second electrical elements, respectively, so they follow the laterally shifting positions of the longitudinal edges of the work. 